1. Field of the Invention
This invention relates to printed polymeric films, more particularly to polymeric films with a polymeric film image printed thereon.
2. Background of the Invention
Short-run printing techniques allow printers and their customers to make a nearly unlimited number of changes to a given printed image and to do so in an essentially instantaneous manner. Thus, such techniques are ideal for customized and/or specialty printing (i.e., where a limited number of pages with a given design, image, text, etc., are to be printed), especially where more than one color is to be included. One such technique is digital printing embodied by, for example, the DCP-1 web press (Xeikon; Mortsel, Belgium) and the E-Print.TM. 1000 digital offset press (Indigo N.V.; Maastricht, The Netherlands).
Recently, short-run printing methods have been adapted for use with flexible packaging materials, particularly polymeric films. Such films typically are in the form of continuous webs rather than discrete sheets. New digital presses designed specifically for use with polymeric films were developed. One example of such a press is the Omnius.TM. color press (Indigo N.V.).
Despite the fact that such film printing presses have been developed, the surface layers of such films (where printing is to occur) have had to be primed prior to printing. For example, one reviewer of this technology has stated, "The Indigo system has been printed on various films, but to provide good adhesion, a surface primer or film-surface modification is necessary." Podhajny, "Technical Report: Revealing the mystery behind digital printing," Converting Magazine, October 1996 at 78. Although surface modification techniques (e.g., flame or corona treatment, buffing, etc.) can be used to prepare the surface of a polymeric film for printing, application of a chemical primer coating more commonly is used.
Polymeric film substrates commonly used with digital color presses such as, for example, the Omnius.TM. color press, include polyesters (3M; St. Paul, Minn.) and oriented polypropylenes (Mobil Chemical Co.; Macedon, N.Y.). Both of these, as well as other commercially available films for use with such printers, require the application of a primer prior to printing, however.
To further complicate the issue, many polymeric films are heat treated (e.g., heat shrunk) prior to end use. Such treatment can occur in a hot water (e.g., 85.degree. C. or higher) bath, a hot air (e.g., about 140.degree. C. or higher) tunnel, or a steam tunnel. Unfortunately, heating of printed polymeric films often causes the printed image to delaminate from the film. This can be due to the effect of entrained solvents softening the ink system, thereby lowering the adherence of the ink to the film. This lowered adherence renders the printed film susceptible to abrasion and/or transfer of the printed image to another surface. In severe cases, the ink can lift entirely away from the substrate.
Use of an unprimed or untreated polymeric film substrate, particularly one which is useful for the packaging of food and which can maintain good adhesion with the image even when heated, in a color printing process has not been described previously.